What is DNA and what does a gene actually do?
DNA is a molecule storing instructions for building and running an organism. A gene is a segment of it that codes for a functional product — usually a protein.
The structure. DNA is a double helix of two strands, each a backbone with bases attached: adenine, thymine, guanine and cytosine. The strands pair specifically — A with T, G with C — which is the key property. Because pairing is fixed, each strand carries the information to reconstruct the other, and that is how DNA is copied before a cell divides.
What a gene does. The classical account is the central dogma:
Transcription. The gene's sequence is copied into messenger RNA.
Translation. Ribosomes read the mRNA in three-base groups called codons, each specifying one amino acid, and assemble them into a chain.
Folding. The chain folds into a three-dimensional protein, and the shape determines what it does — enzymes, structural components, signalling molecules, antibodies.
So a gene is, in essence, a recipe for a protein, and proteins do nearly all the work in a cell.
Where the simple picture breaks down, and this matters:
Most DNA does not code for proteins. Only around 1–2% of the human genome is protein-coding. Much of the rest is regulatory — switches determining which genes are active, in which cells, at what time. Regulation explains why a liver cell and a neuron share identical DNA and behave completely differently.
One gene can make several proteins through alternative splicing, which is why humans have far fewer genes than expected — around 20,000, comparable to a nematode worm.
Many genes produce functional RNA rather than protein.
Epigenetics. Chemical modifications alter gene activity without changing the sequence, and some are heritable.
Most traits are polygenic, influenced by many genes plus environment — single-gene determinism is the exception.